There are many applications in which it is desirable to carry as much power as possible in an rf transmission line or medium. However, if switching must be performed, the maximum power may be limited by the power-handling capacity of the switching devices.
Typically, transistors connected in shunt or series with the line are used to switch rf power. This provides the necessary switching speed for rf systems. However, transistors and other switching devices inherently have a maximum voltage handling capacity, regardless of their size. Thus, the transmission line voltage is limited by the switch characteristics. Since the rf transmission line power is proportional to the square of the voltage, the voltage limitations of the switching devices severely limit the amount of rf power which can be handled.
RF power could theoretically be increased by decreasing impedance; however, rf line impedance is typically set to standard values, for example, 50 ohms in a microwave line. Thus, decreasing transmission line impedance to increase power would create a hybrid circuit to which additional components would have to be added to make it compatible with standard circuitry. In addition, the larger lines required to accomplish low impedance transmission would make high-speed switching difficult to accomplish.
By employing a number of parallel switching devices, more current can be switched. However, if more current is transmitted, the line voltage must be increased proportionally to keep the impedance at the desired (50 ohm) level. Thus, maximum rf power is still limited by the maximum voltage that can be handled by the switches. As a result, to date the monolithic or discrete-transistor switching devices commonly used in rf circuits have kept the maximum transmittable power levels relatively low.